Driving a bi-stable matrix display device

ABSTRACT

In a first display mode, only the information in a first sub-area (W 1 ) of the display screen of a bi-stable matrix display ( 100 ) has to be updated. In a second display mode, the information in a second sub-area (W 2 ) of the display screen has to be updated. The information in the first sub-area (W 1 ) is displayed using optical states which require first drive voltage waveforms (DV 1 ) having a maximum duration equal to a first image update period (IUP 1 ). The information in the second area (W 2 ) is displayed using optical states which require second drive voltage waveforms (DV 2 ) having a maximum duration equal to a second image update period (IUP 2 ). The optical states allowed to be used during the first mode are selected to obtain a first image update period (IUP 1 ) which is shorter than the second image update period (IUP 2 ). In this manner, the refresh rate of the information in the first area (W 1 ) is higher than the refresh rate in the second area (W 2 ).

The invention relates to a drive circuit for driving a bi-stable matrixdisplay device, to a display apparatus comprising a bi-stable matrixdisplay device and such a drive circuit, and to a method of driving abi-stable matrix display device.

Bi-stable matrix display devices, such as, for example, electrophoreticdisplays are used in, for example, electronic books, mobile telephones,personal digital assistants, laptop computers, and monitors.

An electrophoretic display device is known from international patentapplication WO 99/53373. This patent application discloses an electronicink display which comprises two substrates, one of which is transparent,the other substrate is provided with electrodes arranged in rows andcolumns. Display elements or pixels are associated with intersections ofthe row and column electrodes. Each display element is coupled to thecolumn electrode via a main electrode of a thin-film transistor (furtherreferred to as TFT). A gate of the TFT is coupled to the row electrode.This arrangement of display elements, TFT's and row and columnelectrodes jointly forms an active matrix display device.

Each pixel comprises a pixel electrode which is the electrode of thepixel which is connected via the TFT to the column electrodes. During animage update period or image refresh period, a row driver is controlledto select all the rows of display elements one by one, and the columndriver is controlled to supply data signals in parallel to the selectedrow of display elements via the column electrodes and the TFT's. Thedata signals correspond to image data to be displayed.

Furthermore, an electronic ink is provided between the pixel electrodeand a common electrode provided on the transparent substrate. Theelectronic ink is thus sandwiched between the common electrode and thepixel electrodes. The electronic ink comprises multiple microcapsules ofabout 10 to 50 microns. Each microcapsule comprises positively chargedwhite particles and negatively charged black particles suspended in afluid. When a positive voltage is applied to the pixel electrode withrespect to the common electrode, the white particles move to the side ofthe microcapsule directed to the transparent substrate, and the displayelement appears white to a viewer. Simultaneously, the black particlesmove to the pixel electrode at the opposite side of the microcapsulewhere they are hidden from the viewer. By applying a negative voltage tothe pixel electrode with respect to the common electrode, the blackparticles move to the common electrode at the side of the microcapsuledirected to the transparent substrate, and the display element appearsdark to a viewer. When the electric field is removed, the display deviceremains in the acquired state and exhibits a bi-stable character. Thiselectronic ink display with its black and white particles isparticularly useful as an electronic book.

Grey scales can be created in the display-device by controlling theamount of particles that move to the common electrode at the top of themicrocapsules. For example, the energy of the positive or negativeelectric field, defined as the product of field strength and time ofapplication, controls the amount of particles which move to the top ofthe microcapsules.

From the non-pre-published patent application in accordance toapplicants Ser. No. 10/542,910 which has been filed as European patentapplication 03100133.2 it is known to minimize the image retention byextending the duration of the reset pulse which is applied before thedrive pulse. An over-reset pulse is added to the reset pulse, theover-reset pulse and the reset pulse together, have an energy which islarger than required to bring the pixel into one of two extreme opticalstates. The duration of the over-reset pulse may depend on the requiredtransition between successive optical states of a pixel. Unlessexplicitly mentioned, for the sake of simplicity, the term reset pulsemay cover both the reset pulse without the over-reset pulse or thecombination of the reset pulse and the over-reset pulse. By using thereset pulse, the pixels are first brought into one of two well definedextreme optical states before the drive pulse changes the optical stateof the pixel in accordance with the image to be displayed. This improvesthe accuracy of the grey levels.

For example, if black and white particles are used, the two extremeoptical states are black and white. In the extreme state black, theblack particles are at a position near to the transparent substrate, inthe extreme state white, the white particles are at a position near tothe transparent substrate.

The drive pulse has an energy to change the optical state of the pixelto a desired level which may be in-between the two extreme opticalstates. Also the duration of the drive pulse may depend on the requiredtransition of the optical state.

The non-prepublished patent application Ser. No. 10/542,910 discloses inan embodiment that preset pulses (also referred to as the shaking pulse)precedes the reset pulse. Preferably, the shaking pulse comprises aseries of AC-pulses, however, the shaking pulse may comprise a singlepreset pulse only. Each level (which is one preset pulse) of the shakingpulse has an energy (or a duration if the voltage level is fixed)sufficient to release particles present in one of the extreme positions,but insufficient to enable said particles to reach the other one of theextreme positions. The shaking pulse increases the mobility of theparticles such that the reset pulse has an immediate effect. If theshaking pulse comprises more than one preset pulse, each preset pulsehas the duration of a level of the shaking pulse. For example, if theshaking pulse has successively a high level, a low level and a highlevel, this shaking pulse comprises three preset pulses. If the shakingpulse has a single level, only one preset pulse is present.

The non-prepublished patent application Ser. No. 10/507,812, now U.S.Pat. No. 7,126,577 which has been filed as European patent application02077017.8 is directed to the use of shaking pulses directly precedingthe drive pulses.

The complete voltage waveform which has to be presented to a pixelduring an image update period is referred to as the drive voltagewaveform. The drive voltage waveform usually differs for differentoptical transitions of the pixels.

In all embodiments, during each image update period a drive voltagewaveform is supplied which comprises the same sequence, for example: areset pulse preceding a drive pulse, or a shaking pulse, a reset pulseand a drive pulse, or a shaking pulse, a reset pulse, a shaking pulseand a drive pulse. As different pixels may have to change to differentoptical states, and each pixel may change from any optical state to anyoptical state, the duration of each image update period is determined bythe duration of the longest drive voltage waveform.

The driving of the bi-stable display device in accordance with thepresent invention differs from the driving disclosed in thenon-prepublished patent application Ser. No. 10/542,910 which has beenfiled as European patent application 03100133.2 in that the display hasdifferent display modes. In a first display mode, only the informationin a first sub-area of the display screen (further referred to as thefirst area) has to be updated. In a second display mode, the informationin a second sub-area of the display (further referred to as the secondarea) has to be updated. The information in the first area is displayedusing optical states which require first drive voltage waveforms havinga maximum duration equal to a first image update period. The informationin the second area is displayed using optical states which requiresecond drive voltage waveforms having a maximum duration equal to asecond image update period. The optical states allowed to be used duringthe first mode are selected to obtain a first image update period whichis shorter than the second image update period. In this manner, therefresh rate of the information in the first area is higher than therefresh rate in the second area. It is thus possible to refresh theinformation in the first area at a relatively high rate compared torefreshing the information in the second area or in both the first andthe second area. The higher refresh rate in the first area may beimportant if the information displayed in the first area changes at ahigher rate than the refresh rate possible when using the second imageupdate period. An example of an application is a display apparatus whichis able to show a relatively slowly changing greyscale image in thesecond area (the background area) and which displays two level textinformation in the first area (an window overlaying the background area)which should be updated relatively fast in response to user input.

A first aspect of the invention provides a drive circuit for driving abi-stable matrix display device as claimed in claim 1. A second aspectof the invention provides a display apparatus as claimed is claim 13. Athird aspect of the invention provides a method as claimed in claim 17.Advantageous embodiments are defined in the dependent claims.

In an embodiment in accordance with the invention as defined in claim 2,in the first area the information is displayed by using only the twoextreme optical states. The two extreme optical states can be obtainedaccurately with relatively short drive voltage waveforms which maycontain a reset pulse only. The image update period is relatively shortand a relatively high refresh rate is possible.

In an embodiment in accordance with the invention as defined in claim 3,in the second area information is displayed which is allowed to obtainoptical states in-between the extreme optical states. Now, a grey drivepulse (also referred to as drive pulse) which determines the grey levelstarting from one of the extreme optical states is required. Thus, theimage update time required for the addressing of the second area or boththe first and the second area is relatively long.

Preferably, in the second area information is displayed which is allowedto obtain anyone of the possible optical states available for thedisplay, and thus the second image update period must have the maximumduration.

In an embodiment in accordance with the invention as defined in claims 5or 6 the drive circuit is arranged to drive an electrophoretic display.Such an electrophoretic display may comprise microcapsules which containat least two types of different particles. The different particles havedifferent optical properties, are charged differently, and/or havedifferent mobility. During the first display mode, the first imageupdate periods comprise a reset pulse only. The reset pulse has anenergy sufficient to cause the particles to substantially occupy one ofthe two extreme positions corresponding to one of the two extremeoptical states. During the second display mode, the second image updateperiods comprising successively at least a reset pulse and a drivepulse, wherein the drive pulse determines the intermediate optical stateof a pixel.

Consequently, the first image update period and thus the first refreshtime of the first area will be shorter than the second image updateperiod and thus the second refresh time of the second area because thesecond image update period comprises a drive pulse which is not presentin the first image update period. It is thus possible to display blackand white information with a relatively high refresh rate in the firstarea, and to display greyscale information in the second area with arelatively low refresh rate.

In an embodiment in accordance with the invention as defined in claim 7,the drive circuit is arranged for generating a shaking pulse whichprecedes the reset pulse during the second image update period or duringboth the first and the second image update period. The use of a shakingpulse preceding the reset pulse is disclosed in the non-pre-publishedpatent application Ser. No. 10/542,910. The shaking pulse comprises atleast one preset pulse which has an energy sufficient to release theparticles present in one of the two extreme positions corresponding toone of the extreme optical states but insufficient to enable theparticles to each the other one of the two extreme positionscorresponding the other one of the extreme optical states. The use ofthe shaking pulse improves the gray level reproduction, and thus is morerelevant during the second update period than during the first updateperiod.

In an embodiment in accordance with the invention as defined in claim 8,the drive circuit is arranged for generating during the second displaymode further a further shaking pulse occurring in-between the resetpulse and the drive pulse. The use of a shaking pulse preceding thedrive pulse is disclosed in the non-prepublished patent application Ser.No. 10/507,812, now U.S. Pat. No. 7,126,577. The further shaking pulsewill not be present during the first display mode because no drive pulseis present. The use of the further shaking pulse improves the gray levelreproduction.

In an embodiment in accordance with the invention as defined in claim 9,the drive circuit is arranged for generating during the second displaymode a reset pulse which has an energy larger than required for theparticles to reach one of the two extreme positions. The use of such atoo long reset pulse is disclosed in the non-prepublished patentapplication Ser. No. 10/542,910. The use of the longer reset pulse thanrequired improves the gray level reproduction. The use of over-reset ismore relevant during the second image update period which occurs duringthe second display mode than during the first image update period whichoccurs during the first display mode.

In an embodiment in accordance with the invention as defined in claim10, the drive circuit comprises a select driver, a data driver, and acontroller. The controller controls in the first display mode, theselect driver to select lines of pixels corresponding to the first areaonly, and the data driver to supply the first drive voltage waveforms tothe selected ones of the pixels corresponding to the first area only.The controller controls in the second display mode, the select driver toselect lines of pixels corresponding to the second area only or to boththe first and the second area, and the data driver to supply the seconddrive voltage waveforms to the selected ones of the pixels correspondingto the second area only, or to the first and the second area.

During the addressing of the first area, only the optical states of thepixels of the first area may have to be changed. The refresh time orimage update time will become smaller because only a subset of theselect lines have to be selected and because optical states are selectedfor the pixels of the first area which require a maximum image updatetime which is shorter than the image update time required for pixelsoutside the first area.

In an embodiment in accordance with the invention as defined in claim11, the matrix display comprises intersecting select electrodes and dataelectrodes, the pixels are associated with intersections of the selectelectrodes and the data electrodes. The controller controls in the firstdisplay mode, the select driver to supply select voltages to the selectelectrodes associated with the first area only, to select the associatedlines of pixels one by one, and the data driver to supply the firstdrive voltage waveforms to the data electrodes associated with the firstarea only. The controller controls in the second display mode, theselect driver to supply the select voltages to the select electrodesassociated with the second area only, for selecting the associated linesof pixels one by one, and the data driver to supply the second drivevoltage waveforms to the data electrodes associated with the second areaonly.

Thus, during the first display mode, the first area is addressed in thesame manner as usually a complete display would be addressed. Thedifference is that only the select lines associated with the pixels ofthe first area are addressed one by one and that the data must besupplied only to the pixels of the first area to prevent that theoptical state of the pixels of a selected line which pixels are notwithin the first area changes. In the same manner, during the secondmode, only the pixels in the second area are addressed. This preventsthat the pixels in the first area are addressed with the longer imageupdate period required for the pixels of the second area.

The total sequence of fast updating of the image displayed in the firstarea and the slow updating of the image displayed in the second areawill have the minimum duration possible if in the second mode only thepixels in the second area are selected.

It should be noted that it is possible that the first area is composedof several sub areas which form non-overlapping areas. For example, thefirst area comprises a first window in which the user is able to inputcharacters, and a second window in which a list of words is shown whichstart with the input characters. In such an application, the second areapreferably comprises all the pixels which are not in the first andsecond window. The second area may form background information.

In an embodiment in accordance with the invention as defined in claim 12the first area comprises a rectangular window, and the controllerreceives at least coordinates of two opposite corners of the rectangularwindow. The controller determines from the coordinates, in the firstdisplay mode the select electrodes and the data electrodes which areassociated with the first area, and in the second display mode, theselect electrodes and the data electrodes which are associated with thesecond area.

In an embodiment in accordance with the invention as defined in claim15, the display apparatus comprises an electrophoretic display whichcomprises a first and a second type of particles. The different types ofparticles are oppositely charged and have a first and a second color,respectively. The particles are arranged between first and second pixelelectrodes. During the first display mode, the driver circuit supplies,between the first and second pixel electrodes, the first drive voltagewaveform with an energy and polarity to attract the first type ofparticles towards the first pixel electrode and the second type ofparticles towards the second pixel electrode to obtain a first one oftwo extreme optical states showing the first color. Or, the drivercircuit supplies between the first and second pixel electrodes the firstdrive voltage waveform with an energy and polarity to attract the secondtype of particles towards the first pixel electrode and the first typeof particles towards the second pixel electrode to obtain the second oneof the two extreme optical states showing the second color.

In an embodiment in accordance with the invention as defined in claim 16the particles are white and black. Consequently, in this embodiment inaccordance with the invention, the display is able to display a blackand white image in the first area and a grey scale image in the secondarea. This is especially useful if the black and white image is text.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1 shows schematically a display apparatus with a driver and abi-stable display,

FIG. 2 shows different areas on the display screen,

FIG. 3 shows drive voltages used for updating the first or the secondarea on the display screen in accordance with an embodiment of theinvention,

FIG. 4 shows diagrammatically a cross-section of a portion of anelectrophoretic display,

FIG. 5 shows diagrammatically a picture display apparatus with anequivalent circuit diagram of a portion of the electrophoretic display,

FIG. 6 shows drive voltages for updating the second area on the displayscreen in accordance with an embodiment of the invention,

FIG. 7 shows drive voltages for updating the second area on the displayscreen in accordance with an embodiment of the invention,

FIG. 8 shows drive voltages used for updating the first or the secondarea on the display screen in accordance with an embodiment of theinvention, and

FIG. 9 shows a block diagram of a drive circuit for driving thebi-stable display.

In different Figures, the same references are used to indicate the samecomponents or signals.

FIG. 1 shows schematically a display apparatus with a driver 101 and abi-stable matrix display 100. The matrix display 100 comprises pixels 18associated with intersections of the select electrodes 17 and dateelectrodes 11. Usually, the select electrodes 17 extend in the rowdirection and are also referred to as row electrodes and the dataelectrodes 11 extend in the column direction and are also referred to ascolumn electrodes. Usually, the bi-stable matrix display 100 is anactive matrix display comprising transistors which are controlled byselect voltages on the select electrodes 17 to supply data voltages onthe data electrodes 11 to the pixels 18 when a particular row of pixels18 is selected. FIG. 1 indicates a first area W1 on the display screenof the matrix display 100 and a second area W2 on the display screen. Byway of example only, the first area W1 is a rectangular window and thesecond area W2 comprises all the pixels 18 which are not within thewindow W1.

Usually the optical state of the pixels 18 of the complete display 100is updated during an image update period (IUP1, IUP2, see FIG. 3).During an image update period, the rows of pixels 18 are selected one byone. The driver circuit 101 supplies to the select electrodes 17 selectvoltages. The select voltage supplied to the selected row of pixels 18has a level to cause the transistors associated with the selected row ofpixels 18 to conduct. The select voltages supplied to the non-selectedrows of pixels 18 have a level such that the transistors associated withthe non-selected rows of pixels 18 have a high impedance. The drivercircuit 101 further supplies drive voltage waveforms to the pixels 18 ofthe selected row in parallel via the data electrodes 11.

Usually, the drive voltage waveform for a particular pixel 18 depends onthe optical transition to be made by this pixel 18. Especially, theduration of the drive voltage waveform may differ for different opticaltransitions. This is illustrated for a bi-stable display with respect toFIG. 3 and for an electrophoretic display with respect to FIGS. 6 and 7.Because usually all the pixels 18 of the display 100 have to be updated,and because the optical transition of each pixel 18 is arbitrary, theimage update period is determined by the longest image update period.

If only a group of the pixels 18 associated with a sub-area W1 of thedisplay 101 has to be updated and the information to be displayed inthis sub-area W1 does not use optical transitions which require thelongest image update period, it is possible to update the image withinthe sub-area W1 with an image update period shorter than the longestimage update period. Consequently, the refresh rate of the informationdisplayed in the sub-area is higher than would be possible if thelongest image update period was used.

FIG. 2 shows different areas on the display screen. The first area W1now comprises two areas W11 and W12. The second area W2 covers the areaof the display screen not covered by the first area W11, W12. The areaW12 is a rectangular area showing a sequence of characters inputted bythe user. In this example, the user inputted the string fa. The area W11is a rectangular area showing a listing of words starting with thestring fa. The area W2 shows background information, which is, forexample, a comedy book page with grey pictures and text consisting theword “fabulous”, which is not known for the user. The user starts typingfa in W12 and more words starting with fa are listed in W11. The areasW11 and W12 need not be rectangular, but this will complicate theaddressing of the pixels 18 of the areas.

By way of example, the information in the areas or windows W11 and W12is displayed in black and white which are the extreme optical states ofthe display device 101. The information in the area W2 is displayed withgrayscales. The grayscales usually include the two extreme opticalstates black and white and at least one intermediate (grey) state.

The required image update periods IUP1, IUP2 for updating theinformation in the windows W11, W12 and for updating the information inthe window W2, respectively, are elucidated with respect to FIG. 3.

FIG. 3 shows drive voltages used for updating the first or the secondarea on the display screen in accordance with an embodiment of theinvention.

FIG. 3A shows a drive voltage waveform DV1 required to change theoptical state of a pixel 18 from substantially white W to substantiallyblack B. The drive voltage waveform DV1 comprises a reset pulse RE1. Thereset pulse RE1 may have a duration just sufficient to guarantee that,at the end of the reset pulse RE1, the pixel 18 is in the extremeoptical state black B. The reset pulse RE1 may have a longer durationthan this minimally required duration to obtain an over-reset.

FIG. 3B shows a drive voltage waveform DV2 required to change theoptical state of a pixel 18 from substantially white W to anintermediate state dark grey DG. This drive voltage waveform DV2comprises a reset pulse RE2 preceding a drive pulse DP. The reset pulseRE2 may be equal to the reset pulse RE1 and changes the optical state ofthe pixel 18 into substantially black B. The drive pulse DP changes theoptical state from the well defined substantially black B to dark greyDG.

If in the area W2 of the display screen the optical states of the pixels18 should be able to change both from substantially white W tosubstantially black B, and from substantially white W to dark grey DG,the image update period IUP2 during the second display mode isdetermined by the drive voltage waveform DV1, DV2 with the longestduration. Thus the image update period will be IUP2 which has theduration of the reset pulse RE2 and the grey drive pulse DP (alsoreferred to as drive pulse DP) together.

If in the areas W11 and W12 of the display screen the optical states ofthe pixels 18 only need to change to substantially black the imageupdate period IUP1 during the first display mode is determined by thedrive voltage waveform DV1. Thus the image update period will be IUP1which has the duration of the reset pulse RE1 only.

Consequently, if only the image in the areas W11 and W12 is refreshedthe image update period is IUP1 which is shorter than the image updateperiod IUP2 required in the area W2. It is thus possible to refresh theinformation in the areas W11 and W12 at a relatively high rate withrespect to the refresh rate of the information in the area W2. In theapplication shown by way of example in FIG. 2, it is better possible tokeep track with the input of the user.

Usually, the drive voltage waveform DV2 required to reach anintermediate grey level with high accuracy is more complex than shown inFIG. 3B. Such drive voltage waveforms DV2 are shown for anelectrophoretic display in FIGS. 6 and 7. Especially if these complexwaveforms are used to obtain intermediate grey levels during the seconddisplay mode, the image update period may become significantly shorterif only the two extreme optical states are used during the first displaymode.

It is also possible to use another subset of the optical statetransitions than the two extreme optical states to decrease the imageupdate period for information which may be displayed with such a subsetof the optical state transitions. What is relevant is that the durationof the drive voltage waveforms DV1 required for the subset of theoptical state transitions which have to occur within the first area W11,W12 have a duration which is shorter than the duration required for thedrive voltage waveform DV2 for an optical state transition not in thesubset and which optical state may occur within the second area W2.

FIG. 4 shows diagrammatically a cross-section of a portion of anelectrophoretic display, which for example, to increase clarity, has thesize of a few display elements only. The electrophoretic displaycomprises a base substrate 2, an electrophoretic film with an electronicink which is present between two transparent substrates 3 and 4 which,for example, are of polyethylene. One of the substrates 3 is providedwith transparent pixel electrodes 5, 5′ and the other substrate 4 with atransparent counter electrode 6. The counter electrode 6 may also besegmented. The electronic ink comprises multiple microcapsules 7 ofabout 10 to 50 microns. Each microcapsule 7 comprises positively chargedwhite particles 8 and negatively charged black particles 9 suspended ina fluid 40. The dashed material 41 is a polymer binder. The layer 3 isnot necessary, or could be a glue layer. When the pixel voltage VDacross the pixel 18 (see FIG. 1) is supplied as a positive drive voltageVdr (see, for example, FIG. 3) to the pixel electrodes 5, 5′ withrespect to the counter electrode 6, an electric field is generated whichmoves the white particles 8 to the side of the microcapsule 7 directedto the counter electrode 6 and the display element will appear white toa viewer. Simultaneously, the black particles 9 move to the oppositeside of the microcapsule 7 where they are hidden from the viewer. Byapplying a negative drive voltage Vdr between the pixel electrodes 5, 5′and the counter electrode 6, the black particles 9 move to the side ofthe microcapsule 7 directed to the counter electrode 6, and the displayelement will appear dark to a viewer (not shown). When the electricfield is removed, the particles 8,9 remain in the acquired state andthus the display exhibits a bi-stable character and consumessubstantially no power. Electrophoretic media are known per se from e.g.U.S. Pat. No. 5,961,804, U.S. Pat. No. 6,120,839 and U.S. Pat. No.6,130,774 and may be obtained from E-ink Corporation.

FIG. 5 shows diagrammatically a picture display apparatus with anequivalent circuit diagram of a portion of the electrophoretic display.The picture display device 1 comprises an electrophoretic film laminatedon the base substrate 2 provided with active switching elements 19, arow driver 16 and a column driver 10. Preferably, the counter electrode6 is provided on the film comprising the encapsulated electrophoreticink, but, the counter electrode 6 could be alternatively provided on abase substrate if a display operates based on using in-plane electricfields. Usually, the active switching elements 19 are thin-filmtransistors TFT. The display device 1 comprises a matrix of displayelements associated with intersections of row or select electrodes 17and column or data electrodes 11. The row driver 16 consecutivelyselects the row electrodes 17, while the column driver 10 provides datasignals in parallel to the column electrodes 11 to the pixels associatedwith the selected row electrode 17. Preferably, a processor 15 firstlyprocesses incoming data 13 into the data signals to be supplied by thecolumn electrodes 11.

The drive lines 12 carry signals which control the mutualsynchronisation between the column driver 10 and the row driver 16.

The row driver 16 supplies an appropriate select pulse to the gates ofthe TFT's 19 which are connected to the particular row electrode 17 toobtain a low impedance main current path of the associated TFT's 19. Thegates of the TFT's 19 which are connected to the other row electrodes 17receive a voltage such that their main current paths have a highimpedance. The low impedance between the source electrodes 21 and thedrain electrodes of the TFT's allows the data voltages present at thecolumn electrodes 11 to be supplied to the drain electrodes which areconnected to the pixel electrodes 22 of the pixels 18. In this manner, adata signal present at the column electrode 11 is transferred to thepixel electrode 22 of the pixel or display element 18 coupled to thedrain electrode of the TFT if the TFT is selected by an appropriatelevel on its gate. In the embodiment shown, the display device of FIG. 1also comprises an additional capacitor 23 at the location of eachdisplay element 18. This additional capacitor 23 is connected betweenthe pixel electrode 22 and one or more storage capacitor lines 24.Instead of TFTs, other switching elements can be used, such as diodes,MIMs, etc.

FIG. 6 shows drive voltage waveforms across a pixel in differentsituations wherein over-reset is used. By way of example, FIG. 6 arebased on an electrophoretic display with black and white particles andfour optical states: black B, dark grey DG, light grey LG, white W. FIG.6A shows an image update period IUP for a transition from light grey LGor white W to dark grey DG. FIG. 6B shows an image update period IUP fora transition from dark grey DG or black B to dark grey DG. The verticaldotted lines represent the frame periods TF (which usually last 20milliseconds), the line periods occurring within the frame periods TFare not shown. Usually, within one frame period TF all the rows ofpixels 18 are selected one by one.

In both FIG. 6A and FIG. 6B, the pixel voltage VD across a pixel 18comprises successively first shaking pulses SP1, SP1′, a reset pulse RE,RE′, second shaking pulses SP2, SP2′ and a drive pulse Vdr. The drivepulses Vdr occur during the same drive period TD which lasts frominstant t7 to instant t8. The second shaking pulses SP2, SP2′immediately precede the driving pulses Vdr and thus occur during a samesecond shaking period TS2. The reset pulse RE, RE′ immediately precedethe second shaking pulses SP2, SP2′. However, due to the differentduration TR1, TR1′ of the reset pulses RE, RE′, respectively, thestarting instants t3 and t5 of the reset pulses RE, RE′ are different.The first shaking pulses SP1, SP1′ which immediately precede the resetpulses RE, RE′, respectively, thus occur during different first shakingperiods in time TS1, TS1′, respectively.

The second shaking pulses SP2, SP2′ occur for every pixel 18 during asame second shaking period TS2. This will cause a lower powerconsumption if the usual row at a select addressing is applied. But,alternatively this enables to select the duration of this second shakingperiod TS2 much shorter as shown in FIGS. 6A and 6B. For clarity, eachone of levels of the second shaking pulses SP2, SP2′ is present during aframe period TF. In fact, in accordance with the invention, now, duringthe second shaking period TS2, the same voltage levels can be suppliedto all the pixels 18. Thus, instead of selecting the pixels 18 line byline, it is now possible to select all the pixels 18 at once, and only asingle line select period TL (see FIG. 7) suffices per level. Thus, thesecond shaking period TS2 only needs to last four line periods TLinstead of four standard frame periods TF.

Alternatively, it is also possible to change the timing of the drivesignals such that the first shaking pulses SP1 and SP1′ are aligned intime, the second shaking pulses SP2 are then no longer aligned in time(not shown). Now, either the power consumption decreases because of thealigned first shaking pulses SP1 and SP1′, or the first shaking periodTS1 can be much shorter. The power efficiency increases maximally ifboth the first shaking pulses SP1 and SP1′ and the second shaking pulsesSP2 are aligned in time as is shown in FIG. 7.

The driving pulses Vdr are shown to have a constant duration, however,the drive pulses Vdr may have a variable duration.

If the drive method shown in FIGS. 6A and 6B is applied to theelectrophoretic display, outside the second shaking period TS2, thepixels 18 have to be selected line by line by activating the switches 19line by line. The voltages VD across the pixels 18 of the selected lineare supplied via the column electrodes 11 in accordance with the opticalstate the pixel 18 should have. For example, for a pixel 18 in aselected row of which pixel the optical state has to change from white Wto dark grey DG, a positive voltage has to be supplied at the associatedcolumn electrode 11 during the frame period TF starting at instant t0.For a pixel 18 in the selected row of which pixel the optical state hasto change from black B to dark grey DG, a zero voltage has to besupplied at the associated column electrode during the frame period TFlasting from instants t0 to t1.

It is assumed that all optical states (black B, dark grey DG, light greyLG, white W) may occur during the second display mode when the opticalstates of the pixels 18 in the second area W2 are updated. Consequently,the image update period IUP2 during the second display mode isdetermined by the drive voltage waveform with the longest duration. Thedrive voltage waveform with the longest duration is shown in FIG. 6A. Ifduring the first display mode when the optical state of the pixels 18 inthe first area are updated it is not required to be able to make thetransition from white W or light grey LG to dark grey DG, the imageupdate period will not be determined by the relatively long image updateperiod IUP shown in FIG. 6A. For example, if only the optical statesblack B and dark grey are used, the image update period will bedetermined by the duration IUP′ of the drive voltage waveform shown inFIG. 6B which is much shorter than the duration IUP of the drive voltagewaveform shown in FIG. 6A. Consequently, the refresh rate of theinformation displayed in the first area W1 is much higher than therefresh rate of the information displayed in the second area W2.

FIG. 7 shows drive voltages for updating the second area on the displayscreen in accordance with an embodiment of the invention. FIG. 7 showdrive waveforms for all optical transitions to dark grey DG if the drivevoltages VD across a pixel 18 comprise shaking periods SP1, SP2 whichoccur during the same time periods and no over-reset is used.Alternatively, over-reset may be used, or drive voltage waveforms may beused in which the end of the first shaking pulses SP1 and the start ofthe reset pulses RE substantially coincide, in the same manner as shownin FIG. 6. In the latter case, the duration of the image update periodIUP will be dependent on the optical transition and it will not bepossible to align both the shaking pulses SP1 and the shaking pulses SP2in drive waveforms for different optical state transitions.

The use of both a shaking pulse SP1 preceding the reset pulse RE and ashaking pulse SP2 in-between the reset pulse RE and the drive pulse DPimproves the reproducibility of grayscales. The grayscales will be lessinfluenced by the history of the drive voltage. The alignment of theshaking pulses SP1 and SP2 such that they occur at the same time duringeach image update period IUP2 independent on the optical transitionrequired has the advantage that the power efficiency increases. This,because it is possible, for each preset pulse of the shaking pulse SP1,SP2 to select all the lines of pixels 18 simultaneously and to supplythe same data signal level to all the pixels 18. The effect ofcapacitances between pixels 18 and electrodes 11, 17 will decrease.Further, as all the pixels 18 may be selected simultaneously, theduration of the preset pulses of the shaking pulse SP1, SP2 may becomemuch shorter than the standard frame period TF thus shortening the imageupdate period IUP2. This is disclosed in more detail in thenon-prepublished patent application Ser. No. 10/515,686 which has beenfiled as patent application IB03/01983 (WO).

FIG. 7A shows a waveform required to change the optical state of thepixel 18 from white W to dark grey DG. FIG. 7B shows a waveform requiredto change the optical state of the pixel 18 from light grey LG to darkgrey DG. FIG. 7C shows a waveform required to keep the optical state ofthe pixel 18 dark grey DG. FIG. 7D shows a waveform required to changethe optical state of the pixel 18 from black B to dark grey DG. Forother transitions similar drive waveforms are required. For example, forthe transition from white W to black B, portions of the waveform of FIG.7A can be used, but with DP=0V.

In all FIG. 7, the first shaking pulses SP1 occur during the same firstshaking period TS1, the second shaking pulses SP2 occur during the samesecond shaking period TS2, and the driving pulse DP occurs during thesame drive period TD. The driving pulses DP may have differentdurations. The reset pulse RE has a length which depends on the opticaltransition of the pixel 18. For example, in a pulse width modulateddriving, the full reset pulse width TR is required for resetting thepixels 18 from white W to black B or white W to dark grey DG, see FIG.7A. For resetting the pixels 18 from light grey LG to black B or fromlight grey LG to dark grey DG, only ⅔ of the duration of this full resetpulse width TR is required, see FIG. 7B. For resetting the pixels 18from dark grey DG to black B or to dark grey DG, only ⅓ of the durationof this full reset pulse width TR is required, see FIG. 7C. Forresetting the pixels 18 from black B to dark grey DG, no reset pulse REis required, see FIG. 7D.

These waveforms are also useful when the known transition matrix baseddriving methods are used in which previous images are considered indetermining the impulses (time×voltage) for a next image. Alternatively,these waveforms are also useful when the electrophoretic material usedin the display is less sensitive to the image history and/or dwell time.

Thus, to conclude, independent of the duration of the reset pulse RE,the first shaking pulses SP1 and the second shaking pulses SP2 can besupplied to all the pixels 18 simultaneously, which has the advantagesmentioned before.

It has to be noted that in such a display which is able to display theoptical states black B, dark grey DG, light grey LG and white W, theimage update period IUP2 has always the same duration. However, in sucha display apparatus which is optimized to display accurate grey levelsthe image update period IUP2 is relatively long. The present inventionis based on the insight that if on a particular sub-area W11, W12 of thedisplay screen information is displayed for which it is not required touse all the available optical states it is possible to select stateswhich require a shorter image update period IUP1.

For example, if still a high accuracy of the optical states in thesub-area W11, W12 is required, preferably, only the states black B andwhite W are selected. In the sub-area W11, W12 now the drive voltagewaveform DV1 shown in FIG. 8B may be used. For image updates in the areaW2 the much longer lasting voltage waveforms shown in FIG. 7 are used.

FIG. 8 shows drive voltages used for updating the first or the secondarea on the display screen in accordance with an embodiment of theinvention.

FIG. 8A shows a drive voltage waveform DV2 which is identical to thedrive voltage waveform shown in FIG. 7A with over-reset. The verticalarrow indicated by B indicates the instant when the optical state blackB is reached. The reset pulse to the right hand side of this arrowindicates the over-reset. The total duration of this waveform DV2 isIUP2. The drive voltage waveforms for other optical transitions may beidentical to the waveforms shown in FIG. 6 or 7.

FIG. 8B shows a drive voltage waveform DV1 which comprises a shakingpulse SP1 preceding a reset pulse RE1. The total duration of thiswaveform DV1 is IUP1.

If an optimal performance of display of the grayscales is required inthe second area W2, in this area W2 the drive voltage waveforms shown inFIG. 6, 7 or 8A have to be used. The image update period will bedetermined by the drive voltage waveform which has the longest durationand thus is equal to IUP2. If the waveforms of FIG. 7 are used all thewaveforms have a duration IUP2 and thus the image update period is IUP2.If the waveforms of FIG. 6 (no over-reset) or the waveforms based onover-reset shown in FIG. 8A are used, the duration of all the waveformsis IUP (no over-reset) or IUP2 (over-reset), respectively. Consequently,the image update periods are IUP and IUP2, respectively. If thewaveforms are used in which the non used time between the shaking pulsesSP1 and the reset pulse RE1 is eliminated (as shown in FIG. 6), thedrive voltage waveforms have different durations for different opticaltransitions. But, still the image update periods are IUP and IUP2,respectively, as the longest waveform determines the image update periodto be used.

If in the first area W11, W12 only black B and white W have to bedisplayed, the drive voltage waveform DV1 shown in FIG. 8B can be used,and the image update period is IUP1 which is much shorter than the imageupdate periods IUP or IUP2. The waveform DV1 does not require the drivepulse DP and the shaking pulse SP2 preceding the drive pulse DP.Further, if, preferably, the waveforms DV2 for reaching intermediateoptical states comprise an over-reset, no over-reset is required for thetransition to black B. It is even possible to omit the shaking pulse SP1in FIG. 8B as was already shown in FIG. 3A.

To conclude, if only the information in the first area W11, W12 isupdated, a relatively short image update period IUP1 can be used becausethis information can be displayed with a sub-set of the optical stateswhich are selected such that the drive voltage waveforms DV1 do not havethe maximum duration. If the information in the second area W2 isupdated, optical states which require for their transitions long drivevoltage waveforms need to be available. Consequently, the image updateperiod will be relatively large.

In the most practical embodiment in accordance with the invention, onlythe the extreme optical states are required during the image updates inthe first area W1, and all the optical states are available for theinformation to be displayed in the second area W2. The two extremeoptical states can be obtained with a high accuracy with relativelyshort drive voltage waveforms, while the intermediate optical stateswhich may be used in the second area require relatively long drivevoltage waveforms to reach a practically usable accuracy of thegrayscales.

FIG. 9 shows a block diagram of a drive circuit for driving thebi-stable display. The controller/driver 203 receives information ongrayscale drive voltage waveforms 201 and the black and white drivevoltage waveform 202 as stored in a table look up memory 200. Thecontroller/driver 203 further receives the coordinates x1, y1 and x2, y2of two opposing corners of the window W1 on the display screen of thedisplay device 100. The window W1 is the first area, the second area W2comprises the pixels not within the first area W1.

In the first display mode when only the pixels 18 of the first area W1are updated, the controller/driver 203 selects the rows of pixels 18within the first area W1 one by one while the black and white drivevoltage waveforms 202 are supplied via the column electrodes 11 to thepixels 18 within the first area W1 only to prevent a change of theoptical state of pixels 18 outside the first area W1.

In the second display mode when only the pixels 18 of the second area W2are updated, the controller/driver 203 selects the rows of pixels 18within the second area W2 one by one while the grayscale drive voltagewaveforms 201 are supplied via the column electrodes 11 to the pixels 18within the second area W2 only to prevent a change of the optical stateof pixels 18 outside the second area W2.

In the example shown in FIG. 9, during the first display mode the rowsof pixels between the vertical coordinates y1 and y2 are selected, whileduring the second display mode all the rows have to be selected.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. Although, the embodiments areillustrated in more detail with respect to electrophoretic displays, thesame approach may be valid for other bi-stable displays.

In the claims, any reference signs placed between parentheses shall notbe construed as limiting the claim. Use of the verb “comprise” and itsconjugations does not exclude the presence of elements or steps otherthan those stated in a claim. The article “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.The invention may be implemented by means of hardware comprising severaldistinct elements, and by means of a suitably programmed computer. Inthe device claim enumerating several means, several of these means maybe embodied by one and the same item of hardware. The mere fact thatcertain measures are recited in mutually different dependent claims doesnot indicate that a combination of these measures cannot be used toadvantage.

1. A drive circuit (101) for driving a bi-stable matrix display devicehaving a display screen (100) comprising a matrix of pixels (18), in afirst display mode, the drive circuit (101) is arranged for generatingduring first image update periods (IUP1) first drive voltage waveforms(DV1) to allow a first set of optical states, and to supply the firstdrive voltage waveforms (DV1) to a first subset of the pixels (18)covering a first area (W1) of the display screen (100) only, in a seconddisplay mode, the drive circuit (101) is arranged for generating duringsecond image update periods (IUP2) second drive voltage waveforms (DV2)to allow a second set of optical states, and to supply the second drivevoltage waveforms (DV2) to a second subset of the pixels covering asecond area (W2) of the display screen (100), wherein the optical statesof the first set of optical states and of the second set of opticalstates are selected to obtain first image update periods (IUP1) beingshorter than the second image update periods (IUP2).
 2. A drive circuit(101) as claimed in claim 1, wherein in the first display mode, thedrive circuit (100) is arranged for generating during the first imageupdate periods (IUP1) the first drive voltage waveforms (DV1) to obtaintwo extreme optical states only.
 3. A drive circuit (101) as claimed inclaim 1, wherein in a second display mode, the drive circuit (101) isarranged for generating during the second image update periods (IUP2)the second drive voltage waveforms (DV2) to display an image having atleast one optical state in-between the two extreme optical states,wherein at least one of the second drive voltage waveforms (DV2)comprises a grey drive pulse (DP) which is not present during the firstimage update periods (IUP1).
 4. A drive circuit (101) as claimed inclaim 2, wherein in a second display mode, the drive circuit (101) isarranged for generating during the second image update periods (IUP2)the second drive voltage waveforms (DV2) to display an image having atleast one optical state in-between the two extreme optical states,wherein at least one of the second drive voltage waveforms (DV2)comprises a grey drive pulse (DP) which is not present during the firstimage update periods (IUP1).
 5. A drive circuit (101) as claimed inclaim 1, wherein the drive circuit (101) is arranged for driving abi-stable matrix display, wherein the bi-stable display (101) is anelectrophoretic display.
 6. A drive circuit (101) as claimed in claim 4,wherein the drive circuit (101) is arranged for driving anelectrophoretic display comprising microcapsules (7) with at least twotypes of different particles (8, 9), and for generating: during thefirst display mode the first image update periods (IUP1) comprising areset pulse (RE) only, wherein the reset pulse (RE1) has an energyenabling said particles (8, 9) to substantially occupy one of two limitpositions corresponding to one of the extreme optical states, during thesecond display mode at least one of the second image update periods(IUP2) comprising successively at least a reset pulse (RE2) and a greydrive pulse (DP), wherein the grey drive pulse (DP) determines theoptical state of the pixels (18).
 7. A drive circuit (101) as claimed inclaim 4, wherein the drive circuit (101) is arranged for generating ashaking pulse (SP1) preceding the reset pulse (RE1, RE2) during thesecond image update period (IUP2) or during both the first and thesecond image update period (IUP1, IUP2), the shaking pulse (SP1)comprising at least one preset pulse having an energy sufficient torelease the particles (8, 9) present in one of the limit positionscorresponding to one of the extreme optical states but insufficient toenable said particles (8, 9) to reach the other one of the limitpositions corresponding the other one of the extreme optical states. 8.A drive circuit (101) as claimed in claim 6, wherein the drive circuit(101) is arranged for generating during the second display mode afurther shaking pulse (SP2) occurring in-between the reset pulse (RE2)and the drive pulse (DP).
 9. A drive circuit (101) as claimed in claim6, wherein the drive circuit (101) is arranged for generating during thesecond display mode a further reset pulse (REF) succeeding the firstmentioned reset pulse (RE2) to obtain an over reset pulse having anenergy larger than required for the particles (8, 9) to reach one of theextreme positions.
 10. A drive circuit (101) as claimed in claim 2,wherein the drive circuit (101) comprises a select driver (16), a datadriver (10), and a controller (15) for controlling in the first displaymode, the select driver (16) to select lines of pixels (18)corresponding to the first area (W1) only, and the data driver (10) tosupply the first drive voltage waveforms (DV1) to the selected ones ofthe pixels (18) corresponding to the first area (W1) only, and in thesecond display mode, the select driver (16) to select lines of pixels(18) corresponding to the second area (W2) only or to the first and thesecond area (W1, W2), and the data driver (10) to supply the seconddrive voltage waveforms (DV2) to the selected ones of the pixels (18)corresponding to the second area (W2) only or to the first and thesecond area (W1, W2).
 11. A drive circuit (101) as claimed in claim 10,wherein the matrix display (100) comprises intersecting selectelectrodes (17) and data electrodes (11), the pixels (18) beingassociated with intersections of the select electrodes (17) and the dataelectrodes (11), the controller (15) being arranged for controlling inthe first display mode, the select driver (16) to supply select voltagesto the select electrodes (17) associated with the first area (W1) only,for selecting the associated lines of pixels (18) one by one, and thedata driver (10) to supply the first drive voltage waveforms (DV1) tothe data electrodes (11) associated with the first area (W1) only, andin the second display mode, the select driver (16) to supply the selectvoltages to the select electrodes (17) associated with the second area(W2) only, for selecting the associated lines of pixels (18) one by one,and the data driver (10) to supply the second drive voltage waveforms(DV2) to the data electrodes (11) associated with the second area (W2)only.
 12. A drive circuit (101) as claimed in claim 11, wherein thefirst area (W1) comprises a rectangular window, and the controller (15)is arranged to receive at least coordinates (x1, y1, x2, y2) of twoopposite corners of the rectangular window to determine in the firstdisplay mode, the select electrodes (17) and the data electrodes (11)being associated with the first area (W1), and in the second displaymode, the select electrodes (17) and the data electrodes (11) beingassociated with the second area (W2).
 13. A display apparatus comprisinga bi-stable matrix display device (100) and a drive circuit (101) asclaimed in claim
 1. 14. A display apparatus as claimed in claim 13,wherein the bi-stable display (101) is an electrophoretic display.
 15. Adisplay apparatus as claimed in claim 13, wherein the electrophoreticdisplay comprises a first and a second type of particles (8, 9) havingopposite charges and a first and a second color, respectively, saidparticles (8, 9) being arranged between first and second pixelelectrodes (6, 5, 5′), during the first display mode, the driver circuit(101) is arranged for supplying between the first and second pixelelectrodes (6, 5, 5′) the first drive voltage waveform (DV1) having anenergy and polarity to attract the first type of particles (8) towardsthe first pixel electrode (6) and the second type of particles (9)towards the second pixel electrode (5, 5′) to obtain a first one of twoextreme optical states showing the first color, or for supplying betweenthe first and second pixel electrodes (6, 5, 5′) the first drive voltagewaveform (DV1) having an energy and polarity to attract the second typeof particles (9) towards the first pixel electrode (6) and the firsttype of particles (9) towards the second pixel electrode (5, 5′) toobtain a second one of the two extreme optical states showing the secondcolor.
 16. A display apparatus as claimed in claim 15, wherein the firstcolor is black and the second color is white.
 17. A method of driving abi-stable matrix display having a display screen (100) comprising amatrix of pixels (18), in a first display mode, the method comprisesgenerating (101) during first image update periods (IUP1) first drivevoltage waveforms (DV1) to allow a first set of optical states, andsupplying (101) the first drive voltage waveforms (DV1) to a firstsubset of the pixels (18) covering a first area (W1) of the displayscreen (100) only, in a second display mode, the method comprisesgenerating (101) during second image update periods (IUP2) second drivevoltage waveforms (DV2) to allow a second set of optical states, andsupplying (101) the second drive voltage waveforms (DV2) to a secondsubset of the pixels covering a second area (W2) of the display screen(100), wherein the optical states of the first set of optical states andof the second set of optical states are selected to obtain first imageupdate periods (IUP1) being shorter than the second image update periods(IUP2).